This invention relates to a resonance inverter of the type in which the charge and discharge of a resonance capacitor are controlled by several semiconductor switches.
There has been known a parallel resonance inverter. The known inverter has a parallel resonant circuit composed of a resonance capacitor and a resonance inductor. The inverter further comprises semiconductor switches such as thyristors, which control the charge and discharge of the resonance capacitor. Since this type of the resonant circuit can easily cope with even large load fluctuations, it has widely been used for various types of power source, for example, in induction heating furnaces, etc.
It is necessary for the waveform of the current flowing through the switch to have a predetermined lead phase with respect to the terminal voltage waveform of the resonance capacitor, in order for the resonance converter to operate stably. The lead phase angle of the switch current wave form in relation to the resonance capacitor terminal wave form (hereinafter called simply "lead phase angle"), in a steady state takes a fixed value, which is determined by the switch drive period circuit parameters of the resonant circuit (values of each circuit element), the load impedance, etc. However, the voltages and currents of the elements of the inverter in the build-up of the inverter are different from those in the steady state. For this reason, even if the switch drive time interval is the same as in the steady state, the lead phase angle is greatly different from that in the steady state.
FIGS. 1A to 1E show waveforms illustrating such an operation of the inverter. FIG. 1A shows the waveform of the resonance capacitor terminal voltage Vc. FIGS. 1B and 1C show the waveforms of Vg and Vg', which are applied to the gates of the two thyristors as semiconductor switches. FIGS. 1D and 1E show the waveforms of the anode voltages of these two thyristor V3 and V4. At the time of starting, the amplitude of the resonance capacitor terminal voltage fluctuates greatly (about .+-.50% with reference to the amplitude appearing in the steady state), as illustrated, and the reverse voltage applying time duration varies as shown by .tau.1 to .tau.3, . . . When thyristors are used for switches, if at least one of the reverse voltage applying time durations .tau.1 to .tau.3, . . . is shorter than the reverse recovery time of the thyristor, the thyristor turns on again to stop the inverter operation. The reverse recovery time of the thyristor means a time required for thyristor to recover its reverse characteristic.
To cope with this problem, the conventional inverter employs the technique of presetting the reverse voltage applying time duration .tau. in the steady state to a value with a large margin. By this technique, the thyristor can reliably turn off even for the shortest reverse voltage applying time duration .tau.m of those, which change time to time from the start of the inverter to an instant that the inverter operation settles down in the steady state. However, the reverse voltage applying time duration .tau. thus set is 2 or 3 times as long as the shortest time duration .tau.m, and forces the inverter to operate at high reactive power.
During the period of time from the start of the inverter till the inverter operation settles down in a steady state, the reactive power is especially large and the effective power transferred to the load is small. Therefore, the time interval from the start till the steady state is reached is longer. This fact is problematic for the load requiring a quick rise of the power source voltage.